In commonly owned U.S. Pat. No. 5,472,602 a filter is disclosed having a housing extending along an axis and defining a back filtrate compartment and a front solids compartment spaced therefrom and a filter drum rotatable in the housing about the axis. The drum has a generally cylindrical and perforate outer wall centered on the axis, axially fixed in the filtrate compartment, and having an axially forwardly open front end. It also has an end wall extending transverse to the axis and displaceable axially between a closed position fitting in the front outer-wall end to form a closed interior therewith and an open position spaced axially forward of the outer wall and lying in the solids compartment. The end wall is formed at the axis with a central fill opening. Extension structure fixed on the end wall has an annular rim spaced axially backward from the end wall. An annular liner of a flexible foraminous filter medium has a front edge attached to the outer-wall end and a back edge attached to the extension-structure rim. An actuator can axially displace the end wall and extension structure between the closed and open positions and thereby displace the liner from a normal position inside the drum and extending backward from the front end to the rim to an everted position substantially outside the drum and extending forward in the solids compartment from the front end to the rim. A fill tube extending along the axis has a back end and carries an axially nondisplaceable fill head complementarily engageable in the fill opening and having an annular seal engageable with the fill tube. A bearing supports the fill head on the fill tube for rotation thereon about the axis and a guide supports the fill tube on the housing for movement axially thereof so that, when the end wall moves axially with the fill head engaged in the fill opening, the fill tube moves axially jointly with the end wall. A suspension is supplied through the fill tube and fill opening in the closed position of the end wall and normal position of the fill tube to the interior of the drum.
The fill head is sealed in the fill opening by a simple static seal, e.g. an O-ring. The fill tube is generally sealed with respect to the fill head by one or more dynamic seals. Gland-type seals are used in above-cited U.S. Pat. No. 5,472,602 as well as in German patent document 3,740,411 of Gerteis while other more complex dynamic seals are described in German patent documents 1,073,259 of Mayer, 3,219,686 of Schlieperskotter, and 3,507,819 of Sade as well as in German utility model 8,325,049 and European published patent application WO 92/04982 of Gerteis.
While such dynamic seals function very well, at least at low pressure, they are subject to some wear and, therefore, shed a small amount of particles with use. For most applications the contamination posed by these particles is insignificant, but when such a filter is applied to the production of pharmaceuticals it cannot be tolerated. Furthermore the known dynamic seals either do not work when there is a significant pressure differential across them or wear excessively with a big pressure differential, shedding even more wear particles. Thus putting a pressure head across the filter medium to increase the effectiveness of the filter only increases the wear problems and some times even creates a leak.